An induction heating (IH) type of image heating apparatus is known as an image heating apparatus of this kind. This image heating apparatus generates an eddy current through the action of a magnetic field generated by an induction heating apparatus upon an image heating element, and heats an unfixed image on a recording medium such as transfer paper or an OHP (Over Head Projector) sheet through Joule heating of the image heating element by means of this eddy current.
This IH image heating apparatus has the advantage of higher heat production efficiency and faster fixing speed than an image heating apparatus that uses a halogen lamp as the heat source of the heat-producing section that heats the image heating element. Also, with an image heating apparatus that uses a thin sleeve, belt, or the like, as the image heating element, the thermal capacity of the image heating element is small, and the image heating element can be made to produce heat in a short time, enabling startup responsiveness to be greatly improved.
With an IH image heating apparatus, the image heating element is normally maintained at a predetermined fixing temperature (target temperature) by having power supplied to the heat source controlled by a value calculated from a predetermined control rule in accordance with the temperature detected by a temperature detection section located in contact with or close to the image heating element.
With this PID control, not only is the operation amount of the power control section made proportional to deviation between the temperature detected by the temperature detection section and the target temperature of the image heating element based on the development increase/decrease trend, but a factor proportional to a deviation integral and a factor proportional to a deviation derivative are also taken into consideration in performing control.
Also, temperature information from the temperature detection section is sampled in a certain cycle (sampling cycle), and is incorporated into the control rule for PID control.
With this kind of image heating apparatus, to increase the glossiness of a fixed image, or improve the transparency of a fixed image on an OHP sheet, a slower fixing speed than normal is used. Furthermore, with this kind of image heating apparatus, a slower fixing speed than normal is also employed when using a recording medium such as thick paper that requires a large amount of heat for heat-fixing of an unfixed image.
However, with an IH image heating apparatus, when the power supplied to the heat source is controlled by means of the above-described PID control, if the fixing speed varies according to the type of recording medium undergoing heat-fixing, there is a risk that temperature control of the image heating element will become unstable.
That is to say, the image heating element of an IH image heating apparatus rises in temperature through the supply of a predetermined amount of heat by the heat source, but, since the heat production efficiency of the image heating element is high, when the fixing speed changes the amount of heat received from the heat source also changes. For example, if the fixing speed is halved, the amount of heat received by the image heating element from the heat source approximately doubles. Consequently, in this kind of image heating apparatus, even if the power input to the heat source is fixed, the speed of a rise in temperature of the image heating element increases when the fixing speed is reduced.
Also, with this kind of image heating apparatus, there is a certain time lag between execution of power adjustment as a result of PID control computation and detection of the temperature change of the image heating element that is the result of this control.
Thus, with this kind of image heating apparatus, this time lag is taken into consideration in deciding the sampling time for detected temperature information from the temperature detection section. However, with this kind of image heating apparatus, when the fixing speed changes, this sampling time shifts, and the PID control results cannot be fed back accurately.
Thus, a deficiency of this kind of image heating apparatus is that, since the speed of a rise in temperature of the image heating element and the sampling time change due to a change in the fixing speed, PID control of the amount of power supplied to the heat source cannot be performed optimally, and the temperature of the image heating element fluctuates above and below the target temperature.
That is to say, with an image heating apparatus that performs PID control of the amount of power supplied to the heat source, when the fixing speed is slow, variation of the temperature of the image heating element in response to variation of the supply power is large, and, when the value of PID control proportional gain K is large, the results of computation of the operation amount of a switching element (IGBT: Insulated Gate Bipolar Transistor) due to PID control are prone to swing. Thus, when the fixing speed is slow, the temperature of the image heating element fails to converge to the target temperature due to overshoot and so forth. On the other hand, when the fixing speed is fast, if the value of PID control proportional gain K is small, the operation amount of the switching element cannot keep up with temperature variations of the image heating element due to disturbances.
Thus, a problem with this kind of image heating apparatus is that it is not possible to achieve uniform gloss of a fixed image on a recording medium in-plane or uniform transparency of an image on an OHP sheet due to swings in the temperature of the image heating element as described above. Furthermore, a problem with this kind of image heating apparatus is the occurrence of fixing defects known as hot offset and cold offset if the temperature of the image heating element moves outside a temperature range within which fixing is possible that includes the target temperature.
Thus, an image heating apparatus has been proposed whereby the method of deciding the operation amount of a switching element by means of PID control is varied according to the rotational speed of fixing film acting as the image heating element (see Patent Document 1, for example).
In the image heating apparatus disclosed in Patent Document 1, the slower the fixing speed (the rotational speed of the fixing film), the smaller is the value of PID control proportional gain K. For example, in this image heating apparatus there is a proportional gain K table for three fixing speeds, proportional gain K corresponding to the current fixing speed is referenced from this table in accordance with a drive speed signal, and switching element on/off times are calculated according to the PID control rule. Then, with this image heating apparatus, temperature control of the fixing film is performed by adjusting the time of voltage application to an exciting coil functioning as the heat source by means of these switching element on/off operations. Patent Document 1: Unexamined Japanese Patent Publication No. 2002-169410